1. Field of the Invention.
This invention generally relates to the field of monitoring devices used to measure the composition of emulsions and particularly for using electrical measurements to determine the water content of water and oil emulsions.
2. Background of the Invention
The measurement of water content in emulsions is an important one in a number of process monitoring and control applications. Food processing applications include the measurement of the water content of butter, ice cream, and milk. Similarly, the cosmetic industry uses such measurements to control the production of water-laden facial creams.
The measurement of emulsion water content is particularly important in the oil industry. Most oil wells produce significant amounts of water along with the oil. This water tends to be corrosive and deleterious to processing equipment. Water is expensive to transport, difficult to dispose of; and, unlike oil, it has no value. For these reasons, it is important for oil producers, pipeline companies, and refiners to carefully monitor the water content of crude oil.
Many devices based on electrical measurements have been proposed and are being used for determining the water content of oil/water mixtures. Most of the prior art devices have significant limitations. These limitations tend to be caused by the challenging electrical properties of oil/water emulsions. Oil and water emulsions have two distinct forms. In one form, the oil is the continuous phase of the mixture and water is dispersed in the oil as droplets. In the second form, water is the continuous phase and oil is dispersed as droplets within it. The electrical properties of these two forms are quite different even if the water content is identical. The result is that many existing devices operate over a limited range of water content because they cannot measure the electrical properties of both emulsion types. Other devices do not distinguish the emulsion type properly in determining the water content resulting in poor accuracy. Still others attempt to determine the emulsion type so as to accurately determine water content from 0-100%, but do so with methods that are not applicable over the entire range of applications.
The most commonly used measurement devices for these applications are capacitance devices that measure the dielectric constant of the mixture to determine water content. Examples of capacitance devices are disclosed in U.S. Pat. No. 4,266,425 to Allport et al., U.S. Pat. No. 3,523,245 to Love et al., U.S. Pat. No. 3,368,147 to Graham, and U.S. Pat. No. 3,006,189 to Warren et al. These devices generally operate only at lower water contents where oil is the continuous phase of the emulsion because they do not function when the mixture is quite conductive. Another type of device that operates only in the oil continuous phase is disclosed in U.S. Pat. No. 4,458,524 to Meador et al.
An example of a device which only works when the oil/water mixture is water continuous is disclosed in U.S. Pat. No. 4,367,440 to Mazzagatti. The disclosed device measures mixture conductivity to determine water content. This method only works when the mixture is sufficiently conductive to perform a reasonable measurement (i.e., when the water is salty and the mixture is water continuous).
Devices that use electrical measurement methods applicable for both the oil-continuous and water-continuous mixtures are disclosed in U.S. Pat. Nos. 4,289,020 and 4,301,400 to Paap; U.S. Pat. No. 4,429,273 to Mazzagatti; U.S. Pat. No. 4,499,418 to Helms et al.; U.S. Pat. No. 5,272,444 to Cox; and European Patent Application No. 87309659.8 of Bentley et al. None of these references disclose a method for determining how to distinguish between oil-continuous and water-continuous emulsions. Without determining emulsion type, the potential accuracy of these devices is reduced.
Devices that can measure 0-100% water and that use some method for distinguishing emulsion type are disclosed in U.S. Pat. No. 4,774,680 to Agar; U.S. Pat. No. 5,101,367 to Agar; U.S. Pat. No. 5,263,363 to Agar; and U.S. Pat. No. 5,503,004 to Agar. The Agar patents all rely on the comparison of a measured electrical property of the mixture with a predetermined value to determine if the mixture is oil or water continuous. The value that would typically be used for the comparison is the mixture conductivity or another value closely related to it such as microwave energy absorption or loss factor. If the water in the oil/water mixture is conductive, as it is in many applications, then the conductivity of a water-continuous emulsion will be much higher than the conductivity of an oil-continuous mixture having the same water content. However, if the water is fresh water which has a low conductivity, this method is not a reliable determinant of emulsion type.
The comparison method taught in the Agar patents could also be applied using a dielectric constant comparison to a threshold value to determine emulsion type. However, this method is harder to apply because the difference between the dielectric constant of oil-continuous and water-continuous emulsions is usually not so large as is the conductivity difference and because the relevant dielectric constants are quite temperature dependent.
Perl, "Complex Microwave Dielectric Properties of Liquids, Solutions and Emulsions", Ph.D. thesis, Illinois Institute of Technology (May 1984) discloses a system for determining the percentages of oil and water in an oil/water sample by measuring the resonant frequency and Q of a resonant cavity containing the sample. The measured Q and resonant frequency are in turn related to the real and imaginary parts, e' and e", of the complex dielectric constant, e*. Perl uses the loss factor (e"/e') to determine the emulsion type (i.e., oil-in-water or water-in-oil) and then to determine volume fractions of the sample.
Chen, "Measurement of Water Content in Oil With Microwave Reflection", East China Petrol. Inst. (vol. 7, no. 3, pp. 376-388, 1983) determines the percentages of oil and water in an oil/water sample by measuring the reflection coefficient using a microwave apparatus coupled to a mixture of oil and water flowing in a pipe. The measured reflection coefficient is directly related to the impedance of the oil and water mixture and the impedance of the mixture in turn is directly related to the dielectric constant of the mixture. Chen also measures the resistivity of the flowing mixture using an inductive coil around the pipe to determine the emulsion type.
Mansour, "Concentration Measurements in Emulsions," Presentation at the 22.sup.nd International Microwave Power Symposium: A Macro View of Microwaves and FR Heatings, Cincinnati, Ohio (Sept. 1987) discloses a system that measures one or more electrical properties of oil/water mixtures, determines from the measured properties if the mixture is oil or water continuous, and determines the water content of the mixture using two different curves (one each for oil and water continuous mixtures) relating the measured electrical properties to water content. Mansour describes storing the different curves in a computer in the form of equations. Finally, Mansour states the need for temperature correction to compensate the measurements for changes that occur as a function of temperature.
Hammer, "Three-Component Flow Measurement in Oil/Gas/Water Mixtures Using Capacitance Transducers", Ph.D. thesis, University of Manchester, Manchester, UK (Dec. 1983) describes a system consisting of: (1) a microcomputer containing equations for oil and water continuous mixtures; (2) a capacitance transducer that measures electrical properties (capacitance and resistance or conductance) of the mixture; (3) a temperature transducer; (4) a flow measuring device; and (5) a display. The system calculates the water content from the measured electrical properties (suitably corrected for temperature), measures the flow rate, and displays the results on a display. In each case, there is a distinct jump when the mixture changes from oil to water continuous, which permits the emulsion type to be determined.
3. Solution to the Problem
To enable emulsion composition monitors utilizing measurements of an electrical property of the mixture to determine composition in a wider range of applications, a simpler and more generally useful method for determining emulsion type is needed. The present invention takes advantage of the limited range of water contents at which both oil-continuous and water-continuous emulsions can exist.
In typical oil field applications, oil-continuous emulsions rarely contain more than 75% water and water-continuous emulsions rarely contain less than 35% water. These represent the extremes. For a given combination of oil and water, the typical overlap in water contents at which the mixture can be either oil-continuous or water-continuous is far less--usually 5% to 15%. Thus for example, if the highest sustainable water content in an oil-continuous mixture is 65%, then the lowest sustainable water content in a water-continuous emulsion is about 50% water.
The prior art includes a number of mathematical relationships between the electrical properties and the component content of emulsions. These mathematical relations give different solutions for oil-continuous and water-continuous emulsions. When calculating the water content from a measured electrical property using such relations, there is a large difference between the result for the oil-continuous and water-continuous results. The difference is much larger than is the range over which the two emulsion types can coexist for a given oil/water mixture. Suppose the calculated oil-continuous result is 65% water and this represents the maximum possible oil-continuous water content. The water-continuous solution calculated from the same measured dielectric constant will be approximately 39%. This is well below the water content at which a water-continuous emulsion could exist. In other words, the oil-continuous solution gives a realistic value, but the water continuous result does not. Thus, it is possible to determine the emulsion type simply by comparing the water contents associated with the oil and water continuous solutions and selecting the most reasonable solution of the two. This approach to determining emulsion type makes it possible to accurately measure water content from 0-100% using devices that measure emulsion electrical properties.
Oil/water mixtures are only one example. The present invention could be used to determine the composition of any emulsion in which the electrical properties of the emulsion differ significantly depending on which of the components is the continuous phase of the mixture.
The present invention could also be utilized with mixtures containing more than two components. For these mixtures too, there are two different composition solutions depending on the emulsion type for the components that make up an emulsion. An example of such an application is multiphase metering in the oil industry. Multiphase meters measure, among other things, the composition of oil, water and gas emulsions. Accurate measurement of these three components is dependent on the determination of the oil/water emulsion type as it is for an oil/water mixture containing no gas. Thus, it should be understood by those skilled in the art that the method and apparatus taught herein could be used within a multicomponent composition monitor such as that taught in U.S. Pat. No. 5,103,181 to Gaisford which discloses a system for measuring electrical properties and temperature.